Need help ! HCG too low?

My sister did a medicated cycle + TI (no trigger) , she got a faint at what she believes to be 12dpo based on OPKs and they checked her blood that day and her HCG came back at 13. They redid 48hrs later at 14dpo and it came back at 55. The nurse told her it's still 'lower than they'd want' and didn't seem that optimistic. Obviously she's freaked out.

From my experience my RE was always looking for over 50 at the 14dpiui during my IUI cycles. Plus her numbers more than doubled so I think it looks like it's progressing?

Any thoughts?